Open-loop phase estimation methods and apparatus for coherent demodulation of phase modulated carriers in mobile channels

Abstract

Open-loop phase estimation methods and apparatus for coherent demodulation on mobile channels is disclosed. A PSK RF modulated signal is received, RF demodulated to obtain inphase and quadrature components thereof, and the inphase and quadrature components sampled and digitized periodically to provide inphase and quadrature components of the signal vector. A phase estimator strips the data contribution from each vector, leaving only a phase error measurement related to the true phase error. That phase error indication is averaged over a fixed window of time to determine an average phase estimate applicable to the signal vector at the middle of the window. Resulting one hundred eighty degree ambiguities are resolved, and then the applicable signal vector is phase corrected by the average phase estimate and result of the ambiguity resolution, with the following demodulation being responsive to the phase of the phase corrected signal vector.

I claim: 
     
       1. In a ground based mobile channel, a method of coherent recovery of phase in a phase shift keyed digital signal modulated on a carrier comprising the steps of: (a) receiving the phase shift keyed signal modulated on a carrier and demodulating the signal using a local oscillator signal as a reference to provide inphase and quadrature components of the demodulated signal;   (b) providing digitized samples of the inphase and quadrature components of the demodulated signal of step (a) representative of the demodulated signal at each bit time to provide digitized inphase and quadrature components of a first vector having a phase responsive to (i) an information-bearing phase component of the received signal, (ii) the phase difference between the local oscillator signal and the carrier of the received signal, and (iii) interference and noise;   (c) squaring the first vector to provide a second vector having twice the phase angle of the first vector;   (d) inverting every other second vector by inverting the second vector every odd bit time;   (e) for each successive bit time n, finding an estimated error vector by taking the square root of the average of an odd number m of successive second vectors ##EQU5## through ##EQU6## using where applicable the inverted vectors of step (d); (f) multiplying the first vector V n  of step (b) for each bit time n and the conjugate of the estimated phase error vector of step (e) for bit time n to provide a third vector;   (g) multiplying the estimated phase error vector of step (e) for bit time n by the conjugate of the estimated phase error vector of step (e) for bit time n-1 to at least find the sign of the real part of the vector product;   (h) decoding the third vector for each bit time to obtain a bit of data, including inverting the respective bit if the real part of the vector product of step (g) is negative.   
     
     
       2. The method of claim 1 wherein the number m is a predetermined number. 
     
     
       3. The method of claim 1 wherein the number m is adaptively determined based on at least one detected characteristic of the mobile channel. 
     
     
       4. The method of claim 1 wherein the mobile channel is a Gaussian minimum shift keying digital communication channel. 
     
     
       5. The method of claim 4 wherein the step of decoding the third vector includes the steps of: (k) for even bit times, determining the bit value based on the real component of the third vector;   (l) for odd bit times, determining the bit value based on the quadrature component of the third vector.   
     
     
       6. The method of claim 5 wherein, for each set of simultaneously taken samples, the estimated error vector for bit time n is computed from the estimated error vector for bit time n-1 by (i) subtracting from the average of step (e) for bit time n-1, the second vector V for bit time ##EQU7## and (ii) adding the second vector V n+ (m-1)/2 prior to taking the square root to find the estimated error vector for bit time n. 
     
     
       7. In a ground based mobile channel, a method of coherent recovery of digital data encoded in the phase of a phase shift keyed signal modulated on a carrier comprising the steps of: (a) receiving the phase shift keyed signal modulated on a carrier and demodulating the signal using a local oscillator signal as a reference to provide inphase and quadrature components of the demodulated signal;   (b) providing digitized samples of the inphase and quadrature components of the demodulated signal of step (a) representative of the demodulated signal at each bit time to provide digitized inphase and quadrature components of a first vector having a phase responsive to (i) an information-bearing phase component of the received signal, (ii) the phase difference between the local oscillator signal and the carrier of the received signal, and (iii) interference and noise;   (c) squaring the first vector to provide a second vector having twice the phase angle of the first vector;   (d) inverting every other second vector by inverting the second vector every odd bit time;   (e) for each successive bit time n, finding an estimated error vector by taking the square root of the average of a number m of second vectors from samples taken before and after the sample for bit time n using where applicable the inverted vectors of step (d);   (f) dividing the first vector V n  of step (b) for each bit time n by the estimated phase error vector of step (e) for bit time n to provide a third vector;   (g) decoding the third vector for each bit time to obtain a bit of data the digital data.   
     
     
       8. The method of claim 7 wherein step (f) is accomplished by multiplying the first vector V n  of step (b) for each bit time n and the conjugate of the estimated phase error vector of step (e) for bit time n to provide the third vector. 
     
     
       9. The method of claim 7 wherein step (g) is accomplished by multiplying the estimated phase error vector of step (e) for bit time n by the conjugate of the estimated phase error vector of step (e) for bit time n-1 to at least find the sign of the real part of the vector product, and detecting the phase of the third vector for each bit time to obtain a bit of data, including inverting the respective bit if the real part of the vector product of the estimated phase error vector for bit time n and the conjugate of the estimated phase error vector is negative. 
     
     
       10. The method of claim 7 wherein the number m is a predetermined number. 
     
     
       11. The method of claim 7 wherein the number m is adaptively determined based on at least one detected characteristic of the mobile channel. 
     
     
       12. The method of claim 7 wherein the estimated error vector is found by taking the square root of the average of a number m of second vectors from samples taken during successive bit times before and successive bit times after the samples for bit time n. 
     
     
       13. The method of claim 7 wherein the estimated error vector is found by taking the square root of the average of a number m of second vectors from samples taken during successive bit times before and successive bit times after the samples for bit time n and includes the samples taken at bit time n. 
     
     
       14. The method of claim 13 wherein, for each set of simultaneously taken samples, the estimated error vector for bit time n is computed from the estimated error vector for bit time n-1 by (i) subtracting from the average of step (e) for bit time n-1, the second vector V for bit time ##EQU8## and (ii) adding the second vector ##EQU9## prior to taking the square root to find the estimated error vector for bit time n. 
     
     
       15. The method of claim 7 wherein the mobile channel is a Gaussian minimum shift keying digital communication channel. 
     
     
       16. The method of claim 15 wherein the step of decoding the third vector includes the steps of: (i) for even bit times, determining the bit value based on the real component of the third vector;   (j) for odd bit times, determining the bit value based on the quadrature component of the third vector.